This patch adds the --detect-renamed option which makes rsync notice files
that either (1) match in size & modify-time (plus the basename, if possible)
or (2) match in size & checksum (when --checksum was also specified) and use
each match as an alternate basis file to speed up the transfer.

The algorithm attempts to scan the receiving-side's files in an efficient
manner.  If --delete[-before] is enabled, we'll take advantage of the
pre-transfer delete pass to prepare any alternate-basis-file matches we
might find.  If --delete-before is not enabled, rsync does the rename scan
during the regular file-sending scan (scanning each directory right before
the generator starts updating files from that dir).  In this latter mode,
rsync might delay the updating of a file (if no alternate-basis match was
yet found) until the full scan of the receiving side is complete, at which
point any delayed files are processed.

I chose to hard-link the alternate-basis files into a ".~tmp~" subdir that
takes advantage of rsync's pre-existing partial-dir logic.  This uses less
memory than trying to keep track of the matches internally, and also allows
any deletions or file-updates to occur normally without interfering with
these alternate-basis discoveries.

To use this patch, run these commands for a successful build:

    patch -p1 <patches/detect-renamed.diff
    ./configure                                 (optional if already run)
    make

TODO:

  We need to never return a match from fattr_find() that has a basis
  file.  This will ensure that we don't try to give a renamed file to
  a file that can't use it, while missing out on giving it to a file
  that could use it.

based-on: 1ddcdaf3f6808ba53aef9e19f630a18808de22ac
diff --git a/compat.c b/compat.c
--- a/compat.c
+++ b/compat.c
@@ -41,6 +41,7 @@ extern int read_batch;
 extern int delay_updates;
 extern int checksum_seed;
 extern int basis_dir_cnt;
+extern int detect_renamed;
 extern int prune_empty_dirs;
 extern int protocol_version;
 extern int protect_args;
@@ -123,6 +124,7 @@ void set_allow_inc_recurse(void)
 		allow_inc_recurse = 0;
 	else if (!am_sender
 	 && (delete_before || delete_after
+	  || detect_renamed
 	  || delay_updates || prune_empty_dirs))
 		allow_inc_recurse = 0;
 	else if (am_server && !local_server
diff --git a/flist.c b/flist.c
--- a/flist.c
+++ b/flist.c
@@ -60,6 +60,7 @@ extern int non_perishable_cnt;
 extern int prune_empty_dirs;
 extern int copy_links;
 extern int copy_unsafe_links;
+extern int detect_renamed;
 extern int protocol_version;
 extern int sanitize_paths;
 extern int munge_symlinks;
@@ -120,6 +121,8 @@ static int64 tmp_dev = -1, tmp_ino;
 #endif
 static char tmp_sum[MAX_DIGEST_LEN];
 
+struct file_list the_fattr_list;
+
 static char empty_sum[MAX_DIGEST_LEN];
 static int flist_count_offset; /* for --delete --progress */
 static int dir_count = 0;
@@ -287,6 +290,45 @@ static int is_excluded(const char *fname, int is_dir, int filter_level)
 	return 0;
 }
 
+static int fattr_compare(struct file_struct **file1, struct file_struct **file2)
+{
+	struct file_struct *f1 = *file1;
+	struct file_struct *f2 = *file2;
+	int64 len1 = F_LENGTH(f1), len2 = F_LENGTH(f2);
+	int diff;
+
+	if (!f1->basename || !S_ISREG(f1->mode) || !len1) {
+		if (!f2->basename || !S_ISREG(f2->mode) || !len2)
+			return 0;
+		return 1;
+	}
+	if (!f2->basename || !S_ISREG(f2->mode) || !len2)
+		return -1;
+
+	/* Don't use diff for values that are longer than an int. */
+	if (len1 != len2)
+		return len1 < len2 ? -1 : 1;
+
+	if (always_checksum) {
+		diff = u_memcmp(F_SUM(f1), F_SUM(f2), checksum_len);
+		if (diff)
+			return diff;
+	} else if (f1->modtime != f2->modtime)
+		return f1->modtime < f2->modtime ? -1 : 1;
+
+	diff = u_strcmp(f1->basename, f2->basename);
+	if (diff)
+		return diff;
+
+	if (f1->dirname == f2->dirname)
+		return 0;
+	if (!f1->dirname)
+		return -1;
+	if (!f2->dirname)
+		return 1;
+	return u_strcmp(f1->dirname, f2->dirname);
+}
+
 static void send_directory(int f, struct file_list *flist,
 			   char *fbuf, int len, int flags);
 
@@ -2421,6 +2463,25 @@ struct file_list *recv_file_list(int f)
 
 	flist_sort_and_clean(flist, relative_paths);
 
+	if (detect_renamed) {
+		int j = flist->used;
+		the_fattr_list.used = j;
+		the_fattr_list.files = new_array(struct file_struct *, j);
+		if (!the_fattr_list.files)
+			out_of_memory("recv_file_list");
+		memcpy(the_fattr_list.files, flist->files,
+		       j * sizeof (struct file_struct *));
+		qsort(the_fattr_list.files, j,
+		      sizeof the_fattr_list.files[0], (int (*)())fattr_compare);
+		the_fattr_list.low = 0;
+		while (j-- > 0) {
+			struct file_struct *fp = the_fattr_list.files[j];
+			if (fp->basename && S_ISREG(fp->mode) && F_LENGTH(fp))
+				break;
+		}
+		the_fattr_list.high = j;
+	}
+
 	if (protocol_version < 30) {
 		/* Recv the io_error flag */
 		if (ignore_errors)
diff --git a/generator.c b/generator.c
--- a/generator.c
+++ b/generator.c
@@ -78,6 +78,7 @@ extern char *basis_dir[MAX_BASIS_DIRS+1];
 extern int compare_dest;
 extern int copy_dest;
 extern int link_dest;
+extern int detect_renamed;
 extern int whole_file;
 extern int list_only;
 extern int read_batch;
@@ -96,6 +97,7 @@ extern char *backup_suffix;
 extern int backup_suffix_len;
 extern struct file_list *cur_flist, *first_flist, *dir_flist;
 extern struct filter_list_struct daemon_filter_list;
+extern struct file_list the_fattr_list;
 
 int ignore_perishable = 0;
 int non_perishable_cnt = 0;
@@ -103,6 +105,7 @@ int maybe_ATTRS_REPORT = 0;
 
 static dev_t dev_zero;
 static int deletion_count = 0; /* used to implement --max-delete */
+static int unexplored_dirs = 1;
 static int deldelay_size = 0, deldelay_cnt = 0;
 static char *deldelay_buf = NULL;
 static int deldelay_fd = -1;
@@ -113,7 +116,7 @@ static int need_retouch_dir_times;
 static int need_retouch_dir_perms;
 static const char *solo_file = NULL;
 
-/* For calling delete_item() and delete_dir_contents(). */
+/* For calling delete_item(), delete_dir_contents(), and delete_in_dir(). */
 #define DEL_NO_UID_WRITE 	(1<<0) /* file/dir has our uid w/o write perm */
 #define DEL_RECURSE		(1<<1) /* if dir, delete all contents */
 #define DEL_DIR_IS_EMPTY	(1<<2) /* internal delete_FUNCTIONS use only */
@@ -122,6 +125,7 @@ static const char *solo_file = NULL;
 #define DEL_FOR_SYMLINK 	(1<<5) /* making room for a replacement symlink */
 #define DEL_FOR_DEVICE		(1<<6) /* making room for a replacement device */
 #define DEL_FOR_SPECIAL 	(1<<7) /* making room for a replacement special */
+#define DEL_NO_DELETIONS	(1<<9) /* just check for renames w/o deleting */
 
 #define DEL_MAKE_ROOM (DEL_FOR_FILE|DEL_FOR_DIR|DEL_FOR_SYMLINK|DEL_FOR_DEVICE|DEL_FOR_SPECIAL)
 
@@ -146,11 +150,121 @@ static int is_backup_file(char *fn)
 	return k > 0 && strcmp(fn+k, backup_suffix) == 0;
 }
 
+/* Search for a regular file that matches either (1) the size & modified
+ * time (plus the basename, if possible) or (2) the size & checksum.  If
+ * we find an exact match down to the dirname, return -1 because we found
+ * an up-to-date file in the transfer, not a renamed file. */
+static int fattr_find(struct file_struct *f, char *fname)
+{
+	int low = the_fattr_list.low, high = the_fattr_list.high;
+	int mid, ok_match = -1, good_match = -1;
+	struct file_struct *fmid;
+	int diff;
+
+	while (low <= high) {
+		mid = (low + high) / 2;
+		fmid = the_fattr_list.files[mid];
+		if (F_LENGTH(fmid) != F_LENGTH(f)) {
+			if (F_LENGTH(fmid) < F_LENGTH(f))
+				low = mid + 1;
+			else
+				high = mid - 1;
+			continue;
+		}
+		if (always_checksum) {
+			/* We use the FLAG_FILE_SENT flag to indicate when we
+			 * have computed the checksum for an entry. */
+			if (!(f->flags & FLAG_FILE_SENT)) {
+				if (fmid->modtime == f->modtime
+				 && f_name_cmp(fmid, f) == 0)
+					return -1; /* assume we can't help */
+				file_checksum(fname, F_SUM(f), F_LENGTH(f));
+				f->flags |= FLAG_FILE_SENT;
+			}
+			diff = u_memcmp(F_SUM(fmid), F_SUM(f), checksum_len);
+			if (diff) {
+				if (diff < 0)
+					low = mid + 1;
+				else
+					high = mid - 1;
+				continue;
+			}
+		} else {
+			if (fmid->modtime != f->modtime) {
+				if (fmid->modtime < f->modtime)
+					low = mid + 1;
+				else
+					high = mid - 1;
+				continue;
+			}
+		}
+		ok_match = mid;
+		diff = u_strcmp(fmid->basename, f->basename);
+		if (diff == 0) {
+			good_match = mid;
+			if (fmid->dirname == f->dirname)
+				return -1; /* file is up-to-date */
+			if (!fmid->dirname) {
+				low = mid + 1;
+				continue;
+			}
+			if (!f->dirname) {
+				high = mid - 1;
+				continue;
+			}
+			diff = u_strcmp(fmid->dirname, f->dirname);
+			if (diff == 0)
+				return -1; /* file is up-to-date */
+		}
+		if (diff < 0)
+			low = mid + 1;
+		else
+			high = mid - 1;
+	}
+
+	return good_match >= 0 ? good_match : ok_match;
+}
+
+static void look_for_rename(struct file_struct *file, char *fname)
+{
+	struct file_struct *fp;
+	char *partialptr, *fn;
+	STRUCT_STAT st;
+	int ndx;
+
+	if (!partial_dir || (ndx = fattr_find(file, fname)) < 0)
+		return;
+
+	fp = the_fattr_list.files[ndx];
+	fn = f_name(fp, NULL);
+	/* We don't provide an alternate-basis file if there is a basis file. */
+	if (link_stat(fn, &st, 0) == 0)
+		return;
+
+	if (!dry_run) {
+		if ((partialptr = partial_dir_fname(fn)) == NULL
+		 || !handle_partial_dir(partialptr, PDIR_CREATE))
+			return;
+		/* We only use the file if we can hard-link it into our tmp dir. */
+		if (link(fname, partialptr) != 0) {
+			if (errno != EEXIST)
+				handle_partial_dir(partialptr, PDIR_DELETE);
+			return;
+		}
+	}
+
+	/* I think this falls into the -vv category with "%s is uptodate", etc. */
+	if (verbose > 1)
+		rprintf(FINFO, "found renamed: %s => %s\n", fname, fn);
+}
+
 /* Delete a file or directory.  If DEL_RECURSE is set in the flags, this will
  * delete recursively.
  *
  * Note that fbuf must point to a MAXPATHLEN buffer if the mode indicates it's
  * a directory! (The buffer is used for recursion, but returned unchanged.)
+ *
+ * Also note: --detect-rename may use this routine with DEL_NO_DELETIONS set!
  */
 static enum delret delete_item(char *fbuf, uint16 mode, uint16 flags)
 {
@@ -177,6 +291,8 @@ static enum delret delete_item(char *fbuf, uint16 mode, uint16 flags)
 			goto check_ret;
 		/* OK: try to delete the directory. */
 	}
+	if (flags & DEL_NO_DELETIONS)
+		return DR_SUCCESS;
 
 	if (!(flags & DEL_MAKE_ROOM) && max_delete >= 0 && ++deletion_count > max_delete)
 		return DR_AT_LIMIT;
@@ -232,6 +348,8 @@ static enum delret delete_item(char *fbuf, uint16 mode, uint16 flags)
  * its contents, otherwise just checks for content.  Returns DR_SUCCESS or
  * DR_NOT_EMPTY.  Note that fname must point to a MAXPATHLEN buffer!  (The
  * buffer is used for recursion, but returned unchanged.)
+ *
+ * Note: --detect-rename may use this routine with DEL_NO_DELETIONS set!
  */
 static enum delret delete_dir_contents(char *fname, uint16 flags)
 {
@@ -251,7 +369,9 @@ static enum delret delete_dir_contents(char *fname, uint16 flags)
 	save_filters = push_local_filters(fname, dlen);
 
 	non_perishable_cnt = 0;
+	file_extra_cnt += SUM_EXTRA_CNT;
 	dirlist = get_dirlist(fname, dlen, 0);
+	file_extra_cnt -= SUM_EXTRA_CNT;
 	ret = non_perishable_cnt ? DR_NOT_EMPTY : DR_SUCCESS;
 
 	if (!dirlist->used)
@@ -291,7 +411,8 @@ static enum delret delete_dir_contents(char *fname, uint16 flags)
 		if (S_ISDIR(fp->mode)) {
 			if (delete_dir_contents(fname, flags | DEL_RECURSE) != DR_SUCCESS)
 				ret = DR_NOT_EMPTY;
-		}
+		} else if (detect_renamed && S_ISREG(fp->mode))
+			look_for_rename(fp, fname);
 		if (delete_item(fname, fp->mode, flags) != DR_SUCCESS)
 			ret = DR_NOT_EMPTY;
 	}
@@ -456,13 +577,19 @@ static void do_delayed_deletions(char *delbuf)
  * all the --delete-WHEN options.  Note that the fbuf pointer must point to a
  * MAXPATHLEN buffer with the name of the directory in it (the functions we
  * call will append names onto the end, but the old dir value will be restored
- * on exit). */
-static void delete_in_dir(char *fbuf, struct file_struct *file, dev_t *fs_dev)
+ * on exit).
+ *
+ * Note:  --detect-rename may use this routine with DEL_NO_DELETIONS set!
+ */
+static void delete_in_dir(char *fbuf, struct file_struct *file, dev_t *fs_dev,
+			  int del_flags)
 {
 	static int already_warned = 0;
 	struct file_list *dirlist;
-	char delbuf[MAXPATHLEN];
-	int dlen, i;
+	char *p, delbuf[MAXPATHLEN];
+	unsigned remainder;
+	int dlen, i, restore_dot = 0;
+	int save_uid_ndx = uid_ndx;
 
 	if (!fbuf) {
 		change_local_filter_dir(NULL, 0, 0);
@@ -476,17 +603,22 @@ static void delete_in_dir(char *fbuf, struct file_struct *file, dev_t *fs_dev)
 		maybe_send_keepalive();
 
 	if (io_error && !ignore_errors) {
-		if (already_warned)
+		if (!already_warned) {
+			rprintf(FINFO,
+			    "IO error encountered -- skipping file deletion\n");
+			already_warned = 1;
+		}
+		if (!detect_renamed)
 			return;
-		rprintf(FINFO,
-			"IO error encountered -- skipping file deletion\n");
-		already_warned = 1;
-		return;
+		del_flags |= DEL_NO_DELETIONS;
 	}
 
 	dlen = strlen(fbuf);
 	change_local_filter_dir(fbuf, dlen, F_DEPTH(file));
 
+	if (detect_renamed)
+		unexplored_dirs--;
+
 	if (one_file_system) {
 		if (file->flags & FLAG_TOP_DIR)
 			filesystem_dev = *fs_dev;
@@ -496,6 +628,14 @@ static void delete_in_dir(char *fbuf, struct file_struct *file, dev_t *fs_dev)
 
 	dirlist = get_dirlist(fbuf, dlen, 0);
 
+	p = fbuf + dlen;
+	if (dlen == 1 && *fbuf == '.') {
+		restore_dot = 1;
+		p = fbuf;
+	} else if (dlen != 1 || *fbuf != '/')
+		*p++ = '/';
+	remainder = MAXPATHLEN - (p - fbuf);
+
 	/* If an item in dirlist is not found in flist, delete it
 	 * from the filesystem. */
 	for (i = dirlist->used; i--; ) {
@@ -508,6 +648,10 @@ static void delete_in_dir(char *fbuf, struct file_struct *file, dev_t *fs_dev)
 					f_name(fp, NULL));
 			continue;
 		}
+		if (detect_renamed && S_ISREG(fp->mode)) {
+			strlcpy(p, fp->basename, remainder);
+			look_for_rename(fp, fbuf);
+		}
 		/* Here we want to match regardless of file type.  Replacement
 		 * of a file with one of another type is handled separately by
 		 * a delete_item call with a DEL_MAKE_ROOM flag. */
@@ -516,14 +660,19 @@ static void delete_in_dir(char *fbuf, struct file_struct *file, dev_t *fs_dev)
 			if (!(fp->mode & S_IWUSR) && !am_root && fp->flags & FLAG_OWNED_BY_US)
 				flags |= DEL_NO_UID_WRITE;
 			f_name(fp, delbuf);
-			if (delete_during == 2) {
-				if (!remember_delete(fp, delbuf, flags))
+			if (delete_during == 2 && !(del_flags & DEL_NO_DELETIONS)) {
+				if (!remember_delete(fp, delbuf, del_flags | flags))
 					break;
 			} else
-				delete_item(delbuf, fp->mode, flags);
-		}
+				delete_item(delbuf, fp->mode, del_flags | flags);
+		} else if (detect_renamed && S_ISDIR(fp->mode))
+			unexplored_dirs++;
 	}
 
+	if (restore_dot)
+		fbuf[0] = '.';
+	fbuf[dlen] = '\0';
+
 	flist_free(dirlist);
 }
 
@@ -556,9 +705,9 @@ static void do_delete_pass(void)
 		 || !S_ISDIR(st.st_mode))
 			continue;
 
-		delete_in_dir(fbuf, file, &st.st_dev);
+		delete_in_dir(fbuf, file, &st.st_dev, 0);
 	}
-	delete_in_dir(NULL, NULL, &dev_zero);
+	delete_in_dir(NULL, NULL, &dev_zero, 0);
 
 	if (do_progress && !am_server)
 		rprintf(FINFO, "                    \r");
@@ -1269,6 +1418,7 @@ static void list_file_entry(struct file_struct *f)
 	}
 }
 
+static struct bitbag *delayed_bits = NULL;
 static int phase = 0;
 static int dflt_perms;
 
@@ -1557,9 +1707,12 @@ static void recv_generator(char *fname, struct file_struct *file, int ndx,
 		}
 		else if (delete_during && f_out != -1 && !phase
 		    && !(file->flags & FLAG_MISSING_DIR)) {
-			if (file->flags & FLAG_CONTENT_DIR)
-				delete_in_dir(fname, file, &real_sx.st.st_dev);
-			else
+			if (file->flags & FLAG_CONTENT_DIR) {
+				if (detect_renamed && real_ret != 0)
+					unexplored_dirs++;
+				delete_in_dir(fname, file, &real_sx.st.st_dev,
+					      delete_during < 0 ? DEL_NO_DELETIONS : 0);
+			} else
 				change_local_filter_dir(fname, strlen(fname), F_DEPTH(file));
 		}
 		goto cleanup;
@@ -1843,8 +1996,14 @@ static void recv_generator(char *fname, struct file_struct *file, int ndx,
 			goto cleanup;
 		}
 #endif
-		if (stat_errno == ENOENT)
+		if (stat_errno == ENOENT) {
+			if (detect_renamed && unexplored_dirs > 0
+			 && F_LENGTH(file)) {
+				bitbag_set_bit(delayed_bits, ndx);
+				return;
+			}
 			goto notify_others;
+		}
 		rsyserr(FERROR_XFER, stat_errno, "recv_generator: failed to stat %s",
 			full_fname(fname));
 		goto cleanup;
@@ -2247,6 +2406,12 @@ void generate_files(int f_out, const char *local_name)
 	if (verbose > 2)
 		rprintf(FINFO, "generator starting pid=%ld\n", (long)getpid());
 
+	if (detect_renamed) {
+		delayed_bits = bitbag_create(cur_flist->used);
+		if (!delete_before && !delete_during)
+			delete_during = -1;
+	}
+
 	if (delete_before && !solo_file && cur_flist->used > 0)
 		do_delete_pass();
 	if (delete_during == 2) {
@@ -2257,7 +2422,7 @@ void generate_files(int f_out, const char *local_name)
 	}
 	do_progress = 0;
 
-	if (append_mode > 0 || whole_file < 0)
+	if (append_mode > 0 || detect_renamed || whole_file < 0)
 		whole_file = 0;
 	if (verbose >= 2) {
 		rprintf(FINFO, "delta-transmission %s\n",
@@ -2299,7 +2464,7 @@ void generate_files(int f_out, const char *local_name)
 						dirdev = MAKEDEV(DEV_MAJOR(devp), DEV_MINOR(devp));
 					} else
 						dirdev = MAKEDEV(0, 0);
-					delete_in_dir(fbuf, fp, &dirdev);
+					delete_in_dir(fbuf, fp, &dirdev, 0);
 				} else
 					change_local_filter_dir(fbuf, strlen(fbuf), F_DEPTH(fp));
 			}
@@ -2346,7 +2511,21 @@ void generate_files(int f_out, const char *local_name)
 	} while ((cur_flist = cur_flist->next) != NULL);
 
 	if (delete_during)
-		delete_in_dir(NULL, NULL, &dev_zero);
+		delete_in_dir(NULL, NULL, &dev_zero, 0);
+	if (detect_renamed) {
+		if (delete_during < 0)
+			delete_during = 0;
+		detect_renamed = 0;
+
+		for (i = -1; (i = bitbag_next_bit(delayed_bits, i)) >= 0; ) {
+			struct file_struct *file = cur_flist->files[i];
+			if (local_name)
+				strlcpy(fbuf, local_name, sizeof fbuf);
+			else
+				f_name(file, fbuf);
+			recv_generator(fbuf, file, i, itemizing, code, f_out);
+		}
+	}
 	phase++;
 	if (verbose > 2)
 		rprintf(FINFO, "generate_files phase=%d\n", phase);
diff --git a/options.c b/options.c
--- a/options.c
+++ b/options.c
@@ -81,6 +81,7 @@ int am_sender = 0;
 int am_starting_up = 1;
 int relative_paths = -1;
 int implied_dirs = 1;
+int detect_renamed = 0;
 int numeric_ids = 0;
 int allow_8bit_chars = 0;
 int force_delete = 0;
@@ -390,6 +391,7 @@ void usage(enum logcode F)
   rprintf(F,"     --modify-window=NUM     compare mod-times with reduced accuracy\n");
   rprintf(F," -T, --temp-dir=DIR          create temporary files in directory DIR\n");
   rprintf(F," -y, --fuzzy                 find similar file for basis if no dest file\n");
+  rprintf(F,"     --detect-renamed        try to find renamed files to speed up the transfer\n");
   rprintf(F,"     --compare-dest=DIR      also compare destination files relative to DIR\n");
   rprintf(F,"     --copy-dest=DIR         ... and include copies of unchanged files\n");
   rprintf(F,"     --link-dest=DIR         hardlink to files in DIR when unchanged\n");
@@ -577,6 +579,7 @@ static struct poptOption long_options[] = {
   {"compare-dest",     0,  POPT_ARG_STRING, 0, OPT_COMPARE_DEST, 0, 0 },
   {"copy-dest",        0,  POPT_ARG_STRING, 0, OPT_COPY_DEST, 0, 0 },
   {"link-dest",        0,  POPT_ARG_STRING, 0, OPT_LINK_DEST, 0, 0 },
+  {"detect-renamed",   0,  POPT_ARG_NONE,   &detect_renamed, 0, 0, 0 },
   {"fuzzy",           'y', POPT_ARG_VAL,    &fuzzy_basis, 1, 0, 0 },
   {"no-fuzzy",         0,  POPT_ARG_VAL,    &fuzzy_basis, 0, 0, 0 },
   {"no-y",             0,  POPT_ARG_VAL,    &fuzzy_basis, 0, 0, 0 },
@@ -1601,7 +1604,7 @@ int parse_arguments(int *argc_p, const char ***argv_p)
 		inplace = 1;
 	}
 
-	if (delay_updates && !partial_dir)
+	if ((delay_updates || detect_renamed) && !partial_dir)
 		partial_dir = tmp_partialdir;
 
 	if (inplace) {
@@ -1610,6 +1613,7 @@ int parse_arguments(int *argc_p, const char ***argv_p)
 			snprintf(err_buf, sizeof err_buf,
 				 "--%s cannot be used with --%s\n",
 				 append_mode ? "append" : "inplace",
+				 detect_renamed ? "detect-renamed" :
 				 delay_updates ? "delay-updates" : "partial-dir");
 			return 0;
 		}
@@ -1972,6 +1976,8 @@ void server_options(char **args, int *argc_p)
 			args[ac++] = "--super";
 		if (size_only)
 			args[ac++] = "--size-only";
+		if (detect_renamed)
+			args[ac++] = "--detect-renamed";
 	} else {
 		if (skip_compress) {
 			if (asprintf(&arg, "--skip-compress=%s", skip_compress) < 0)
diff --git a/rsync.yo b/rsync.yo
--- a/rsync.yo
+++ b/rsync.yo
@@ -389,6 +389,7 @@ to the detailed description below for a complete description.  verb(
      --modify-window=NUM     compare mod-times with reduced accuracy
  -T, --temp-dir=DIR          create temporary files in directory DIR
  -y, --fuzzy                 find similar file for basis if no dest file
+     --detect-renamed        try to find renamed files to speed the xfer
      --compare-dest=DIR      also compare received files relative to DIR
      --copy-dest=DIR         ... and include copies of unchanged files
      --link-dest=DIR         hardlink to files in DIR when unchanged
@@ -1561,6 +1562,21 @@ Note that the use of the bf(--delete) option might get rid of any potential
 fuzzy-match files, so either use bf(--delete-after) or specify some
 filename exclusions if you need to prevent this.
 
+dit(bf(--detect-renamed)) With this option, for each new source file
+(call it em(src/S)), rsync looks for a file em(dest/D) anywhere in the
+destination that passes the quick check with em(src/S).  If such a em(dest/D)
+is found, rsync uses it as an alternate basis for transferring em(S).  The
+idea is that if em(src/S) was renamed from em(src/D) (as opposed to em(src/S)
+passing the quick check with em(dest/D) by coincidence), the delta-transfer
+algorithm will find that all the data matches between em(src/S) and em(dest/D),
+and the transfer will be really fast.
+
+By default, alternate-basis files are hard-linked into a directory named
+".~tmp~" in each file's destination directory, but if you've specified
+the bf(--partial-dir) option, that directory will be used instead.  These
+potential alternate-basis files will be removed as the transfer progresses.
+This option conflicts with bf(--inplace) and bf(--append).
+
 dit(bf(--compare-dest=DIR)) This option instructs rsync to use em(DIR) on
 the destination machine as an additional hierarchy to compare destination
 files against doing transfers (if the files are missing in the destination
diff --git a/util.c b/util.c
--- a/util.c
+++ b/util.c
@@ -1178,6 +1178,32 @@ int handle_partial_dir(const char *fname, int create)
 	return 1;
 }
 
+/* We need to supply our own strcmp function for file list comparisons
+ * to ensure that signed/unsigned usage is consistent between machines. */
+int u_strcmp(const char *p1, const char *p2)
+{
+        for ( ; *p1; p1++, p2++) {
+		if (*p1 != *p2)
+			break;
+	}
+
+	return (int)*(uchar*)p1 - (int)*(uchar*)p2;
+}
+
+/* We need a memcmp function compares unsigned-byte values. */
+int u_memcmp(const void *p1, const void *p2, size_t len)
+{
+	const uchar *u1 = p1;
+	const uchar *u2 = p2;
+
+	while (len--) {
+		if (*u1 != *u2)
+			return (int)*u1 - (int)*u2;
+	}
+
+	return 0;
+}
+
 /* Determine if a symlink points outside the current directory tree.
  * This is considered "unsafe" because e.g. when mirroring somebody
  * else's machine it might allow them to establish a symlink to
diff -up a/proto.h b/proto.h
--- a/proto.h
+++ b/proto.h
@@ -362,6 +362,8 @@ char *normalize_path(char *path, BOOL fo
 char *full_fname(const char *fn);
 char *partial_dir_fname(const char *fname);
 int handle_partial_dir(const char *fname, int create);
+int u_strcmp(const char *p1, const char *p2);
+int u_memcmp(const void *p1, const void *p2, size_t len);
 int unsafe_symlink(const char *dest, const char *src);
 char *human_num(int64 num);
 char *human_dnum(double dnum, int decimal_digits);
diff -up a/rsync.1 b/rsync.1
--- a/rsync.1
+++ b/rsync.1
@@ -464,6 +464,7 @@ to the detailed description below for a
      \-\-modify\-window=NUM     compare mod\-times with reduced accuracy
  \-T, \-\-temp\-dir=DIR          create temporary files in directory DIR
  \-y, \-\-fuzzy                 find similar file for basis if no dest file
+     \-\-detect\-renamed        try to find renamed files to speed the xfer
      \-\-compare\-dest=DIR      also compare received files relative to DIR
      \-\-copy\-dest=DIR         ... and include copies of unchanged files
      \-\-link\-dest=DIR         hardlink to files in DIR when unchanged
@@ -1790,6 +1791,22 @@ Note that the use of the \fB\-\-delete\f
 fuzzy\-match files, so either use \fB\-\-delete\-after\fP or specify some
 filename exclusions if you need to prevent this.
 .IP 
+.IP "\fB\-\-detect\-renamed\fP"
+With this option, for each new source file
+(call it \fIsrc/S\fP), rsync looks for a file \fIdest/D\fP anywhere in the
+destination that passes the quick check with \fIsrc/S\fP.  If such a \fIdest/D\fP
+is found, rsync uses it as an alternate basis for transferring \fIS\fP.  The
+idea is that if \fIsrc/S\fP was renamed from \fIsrc/D\fP (as opposed to \fIsrc/S\fP
+passing the quick check with \fIdest/D\fP by coincidence), the delta\-transfer
+algorithm will find that all the data matches between \fIsrc/S\fP and \fIdest/D\fP,
+and the transfer will be really fast.
+.IP 
+By default, alternate\-basis files are hard\-linked into a directory named
+\(dq\&.~tmp~\(dq\& in each file\(cq\&s destination directory, but if you\(cq\&ve specified
+the \fB\-\-partial\-dir\fP option, that directory will be used instead.  These
+potential alternate\-basis files will be removed as the transfer progresses.
+This option conflicts with \fB\-\-inplace\fP and \fB\-\-append\fP.
+.IP 
 .IP "\fB\-\-compare\-dest=DIR\fP"
 This option instructs rsync to use \fIDIR\fP on
 the destination machine as an additional hierarchy to compare destination
